Autoimmune disease is a major healthcare problem and a leading cause of mortality amongst women and young children. Autoreactive B cells play a key role in the development of autoimmunity, predominately by secreting autoantibodies. In healthy individuals, autoreactive B cells are regulated by tolerance mechanisms that include B cell anergy. Failure of B cell anergy represents a driving force in the development of autoimmunity. Despite this, the mechanisms that regulate B cell anergy remain poorly defined. Our long-term goal is to understand the mechanisms that maintain B cell anergy. Regulatory T cells (Tregs) play a key role in immune homeostasis. Studies defining how Tregs regulate dendritic cells and T cells (CD4+ and CD8+) have occurred at a rapid pace. However, studies defining their role in the regulation of other cell types, such as B cells, have been less aggressive. This is surprising since the absence of Tregs results in profound autoimmunity and autoantibody production. We recently reported that the production of autoantibodies, in the absence of Tregs, occurs due to loss of B cell anergy. The objective of this proposal is to define the mechanism(s) by which the absence of Tregs supports loss of B cell anergy. Our preliminary studies indicate that autoantibody production, mediated by the absence of Tregs, is dependent on the presence of CD4+ T cells. In addition, we have shown that the absence of Tregs results in the expansion of follicular helper T cells (TFH cells). We also show that these TFH cells are pathogenic, in that they are sufficient to drive loss of B cell anergy. We therefore hypothesize that expansion of pathogenic TFH cells is a fundamental mechanism responsible for the generation of autoantibody production in situations where there is a relative decrease or absence of Tregs. This proposal will determine how pathogenic TFH cells support loss of B cell anergy and investigate the mechanism responsible for the expansion of pathogenic TFH cells. These studies will help define a novel mechanism by which anergic B cells are regulated, with potential for developing new therapeutic strategies for the treatment of autoimmunity.